1. Field of the Invention
The present invention relates to LED driving technology, and in particular, relates to the driving technology for suppressing LED flickers.
2. Description of the Related Art
Methods for driving LEDs can be classified into two categories: analog driving methods and digital driving methods. The analog driving method adjusts the LED brightness by changing the LED current. However, this method is not precise enough for brightness control, and it may change the LED's light color at the same time. Therefore, most LED products nowadays are driven digitally.
The digital driving method adjusts LED brightness via pulse width modulation (PWM). FIG. 1 shows the relationship between a system clock signal clk and a 4-bit brightness control signal SLC. As is well known in the art, 4 bits can be used to represent 24 brightness values. Thus, in FIG. 1, the PWM cycle of the 4-bit brightness control signal SLC (i.e., the total length of the brightness control signal) is identical to 16 illuminating clocks. The pulse width modulation is achieved by varying the duty cycle of the LED (i.e., by varying the duty ratio of the illuminating duration to the whole duration of the signal). With the PWM, the LED brightness can be digitally controlled. From FIG. 1, it can be known that the duty cycle of the brightness control signal SLC is 11/16, where the pulse width (high level portion in this Figure) has 11 illuminating clocks. In the duration of the aforementioned 11 clocks, the LED is turned on and emits lights. However, in the following 5 clocks, the LED is turned off. The PWM changes the relative lengths of the time that the LED is turned on and off, and thus the brightness of the LED can be adjusted.
Note that when the LED is desired to exhibit low brightness, the number of the continuous non-illuminating pulses which represents that the LED is turned off will increase. Although the PWM cycle is usually extremely short, the LED's winking changes (flickers) will be difficult to be perceived by humans visually; however, they are noticeable when filming the LED through a digital camera having a low image capture rate. Therefore, the present invention provides a digital LED driving technology for reducing flickers.